Dc resistivity, magnetic susceptibility, and Hall and Seebeck coefficients were measured in the 2–400 K range on perovskite ${\text{BaTiO}}_{3-\delta }$ single crystals with electron concentration of $9.8\times {10}^{17}-3.5\times {10}^{20}{\text{cm}}^{-3}$. The insulator-metal transition in ${\text{BaTiO}}_{3-\delta }$ was found at critical electron concentration $n_c\approx 1.6\times {10}^{20}{\text{cm}}^{-3}$. In contrast to ${\text{SrTiO}}_{3-\delta }$, both Hall and Seebeck coefficients of metallic ${\text{BaTiO}}_{3-\delta }$ show strong temperature dependence below 290 K which culminates by a sign reversal from negative to positive. The temperature of the sign reversal anomaly is concentration dependent and increases with $n$. Magnetic susceptibility of metallic samples shows anomalous decrease upon cooling below 300 K. It is proposed that the low-temperature $p$-type conductivity in ${\text{BaTiO}}_{3-\delta }$ may be attributed to the charge-carrier contributions from the two electronic bands separated by a narrow (3–7 meV) energy gap.

• Received 9 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.045107